The present invention relates to a biaxially oriented polyester film with a flame-retardant finish.
Biaxially oriented films made of linear polyesters, in particular of polyethylene terephthalate are widely used for industrial purposes. In one very important respect, these films are employed in electrical engineering, for example, in the manufacture of transformers and wire insulations, as flat strip conductors, for cable and slot insulations and in the manufacture of capacitors.
For reasons of safety, for example, in radio and television engineering or in the construction of high-speed motors, it is desirable to attain higher heat categories for these polyester films and to improve their low-flammability characteristics.
In U.S. Pat. No. 3,941,752, low-flammability linear polyesters of dicarboxylic acid and diol components as well as phosphorus containing chain members are disclosed, which are characterized by the fact that the phosphorus containing chain members are structural units having the formula: ##STR2##
In these chain members, which constitute approximately 3 to 20 mole percent of the acid component of the polyester,
R represents a saturated, open-chain or cyclic alkylene radical, an arylene radical or an aralkylene radical, and PA1 R.sub.1 represents an alkyl group containing up to 6 carbon atoms, an aryl group or an aralkyl group. PA1 R.sub.1 is an alkyl group containing up to 6 carbon atoms, an aryl or an aralkyl group.
Although the abovementioned U.S. Patent indicates that shaped bodies, especially in the form of fibers, may be made of these polyesters, it is not suggested to produce biaxially oriented films for preferred use in electrical engineering applications.